已知:数列{an}的前n项和为Sn,满足Sn=2an-2n(n∈N*)(1)求数列{an}的通项公式an;(2)若数列{bn}满

2025-04-22 16:20:47
推荐回答(1个)
回答1:

(1)当n∈N*时,Sn=2an-2n,①则当n≥2,n∈N*时,Sn-1=2an-1-2(n-1).②
①-②,得an=2an-2an-1-2,即an=2an-1+2,∴an+2=2(an-1+2)∴

an+2
an?1+2
=2.
当n=1 时,S1=2a1-2,则a1=2,当n=2时,a2=6,∴{an+2}是以a1+2为首项,以2为公比的等比数列.
∴an+2=4?2n-1,∴an=2n+1-2,(7分)
(2)由bn=log2(an+2)=log22n+1=n+1,得
bn
an+2
n+1
2n+1

则Tn=
2
22
+
3
23
+…+
n+1
2n+1
,③
1
2
Tn
2
23
+…+
n
2n+1
+
n+1
2n+2
,④
③-④,得
1
2
Tn
2
22
+
1
23
+
1
24
+…+
1
2n+1
?
n+1
2n+2

=
1
4
+
1
4
(1?
1
2n
)
1?
1
2
?
n+1
2n+2

=
1
4
+
1
2
?
1
2n+1
?
n+1